iChat not connecting

iChat is behaving strangely - I can connect and exchange text, but once I connect an iSight camera, nothing works. I can't connect for video chat, and anyone initiating one doesn't get through to my machine. Likewise, I can't initiate video chat with anyone else. The video icon is present in both my and the recipient's buddy list.

This has happened since I recently got a new broadband connection/modem. What setings should I look to check/change to get things working?